Kidney tests for people with diabetes

I recently received an e-mail from a friend who was concerned about interpreting kidney function tests (KFTs), which led me to think it's time for a review of these tests to help you interpret your lab results. There are lots of discussions of these tests on-line, ranging from detailed and frankly verbose (e.g., look up the name of the test at Wikipedia) to straightforward and very simple (e.g., see Lab Tests Online). I won't go into how often these tests should be obtained, nor what the treatment might be if abnormalities are identified; the ADA has published some recommendations in its Position Statement on Diabetic Nephropathy.

Kidney tests that should be considered for people with diabetes fall into two major categories, blood tests and urine tests. It's rare that people with diabetes will need more sophisticated testing such as kidney sonograms or X-rays or kidney biopsy, but on rare occasions, these might be recommended. And sometimes tests that involve urine are sometimes ordered on people with diabetes, such as measurement of urine glucose or urine ketones, but these tests may not divulge any information about the status of the kidneys themselves.

The commonly-ordered KFTs for people with diabetes include:

microalbumin: A urine test to see if the kidneys are "leaking" small amounts of protein. Ordinarily there should be no protein in the urine, but in diabetes, one of the first indicators that something's happening to the kidneys is the presence of small amounts of protein. The microalbumin level can be measured on a "spot" urine (that is, a sample taken any time of day), or can be measured in a 24-hour urine sample.

urinalysis (often abbreviated "UA"): A urine test that may measure lots of different things. Depending on where it's done, a UA will probably include urine glucose level, ketone level, protein level, urine acidity (called pH), density (specific gravity) and presence of blood and/or white blood cells. Sometimes the urine sample is also examined under a microscope to look more closely for whether the urine contains red or white blood cells, bacteria, or crystals.

BUN: The blood urea nitrogen test is a possible indicator of kidney disease when it's elevated, but it also can go up simply from dehydration or even from ingestion of a large protein meal. Normal range is someplace around 10-20, depending on the laboratory. And no, it's not pronounced as the word "bun", but by its initials: "B-U-N."

creatinine: Like the BUN, a indicator of kidney disease when blood levels are elevated. Normal range varies with age and sex; for young adults, levels in men would probably be under 1.2 and young women under 1.0. For older adults, slightly higher levels are considered acceptable. Note: there's another test that is spelled almost the same, creatine, but which measures something else! Although usually done from a blood sample, creatinine is occasionally measured in a urine sample, as part of a creatinine clearance (see below).

creatinine clearance: A test that combines a 24-hour urine sample and a blood test. Sometimes abbreviated CCr or CrCl. It is an approximation of how well the kidneys are removing waste products. CCr is the most common way of measuring the kidneys' filtration rate, known in technical lingo as the Glomerular Filtration Rate (GFR). There are calculations needed to get the CCr result; one online calculator may be found at http://www-users.med.cornell.edu/~spon/picu/calc/crclcalc.htm. Most physicians don't do these calculations, but rely on the laboratory to provide the calculated result and normal range. The normal values for CCr are about 100, plus or minus. The lower the value, the lousier the kidney function; kidneys that have completely failed to process wastes would have a CCr of zero.

24-hour urine protein: This test measures if the kidneys are "leaking" moderate or large amounts of protein into the urine. Ordinarily, there should be no protein in the urine, but in some severe cases of kidney disease, over a gram of protein might be spilled into the urine in 24 hours. If there's no microalbumin, then there's no need to measure the 24-hour urine protein, but contrariwise, if the microalbumin or urine dipstick protein level seems high, then checking the amount of protein excreted into the urine over 24 hours is the next step. For most patients, if a 24-hour urine protein is ordered, a creatinine clearance will be done at the same time.